211 



the ocular region, ventral margin very sligthly sinuated and terminating behind 

 in a somewhat projecting blunt corner, anterior extremity obtusely rounded 

 and slightly bowed below, posterior produced above the middle to a well 

 defined beak-like protuberance; seen dorsally, broadly oval in outline, with 

 the greatest width attaining 2 / 3 of the length, side-edges in their greater extent 

 evenly curved, but distinctly angular behind, anterior extremity obtusely pointed, 

 posterior abruptly contracted to a narrow conical prominence. Valves of rather 

 firm consistency, with the surface very uneven, each valve being provided with 

 68 somewhat irregular and partly interrupted sharp longitudinal folds or ribs, 

 2 of which, located near the ventral face, are particularly strong and terminate 

 behind in dentiform projections; a faint reticulation moreover present, especially 

 visible in the posterior part of the shell. Anterior antennae very slender, with 

 the 2nd joint of the terminal part the longest, last joint a little shorter than 

 the preceding one. Posterior antennae with the dividing suture of the penulti- 

 mate joint located rather above the middle. Legs comparatively slender, last 

 pair with the 1st joint of the terminal part about the lengt of the other 2 

 combined. 



Male of nearly same size as female, but having the shell somewhat 

 narrower, with the posterior extremity more obliquely truncated bebw. Copu- 

 lative appendages with the basal part oval in shape and sending off below a 

 knife-schaped recurved piece, copulatory string not much prolonged, but con- 

 siderably thickened at the base; terminal part produced at the end to a beak- 

 shaped projection and having the anterior edge bowed in the middle. 



Colour of shell light brownish or violaceous. 



Length of adult female 0.52 mm. 



Remarks. The present species is easily recognised from the other indi- 

 genous forms by the short and tumid shell and by the sharp longitudinal ribs 

 adorning the valves and partly terminating behind in dentiform projections. 

 Some of the forms recorded by G. W. Miiller from the Gulf of Naples come 

 very near to this species, though none of them are apparently strictly identical 

 with the northern form. 



Occurrence. I have taken this form not unfrequently in several places of 

 our south and west coasts, from the Christiania Fjord to Trondhjem, at depths 

 ranging from 6 to 20 fathoms, sandy bottom. 



Distribution. British Isles, ? Mediterranean. 



Fossil. Norway, British Isles. 



